I. Field of the Invention
The present invention relates to a barium ferrite magnetic powder used for a coating type high-density magnetic recording medium, a method of manufacturing thereof, and a high-density magnetic recording medium using the barium ferrite magnetic powder.
II. Description of the Prior Art
A conventional magnetic recording medium comprises a nonmagnetic support of polyethylene terephthalate, and a magnetic layer containing a fine magnetic powder and a binder as major constituents, formed on the nonmagnetic support. A conventional fine magnetic powder comprises needle-like magnetic particles selected from .gamma.-Fe.sub.2 O.sub.3, CrO.sub.2, Co-.gamma.Fe.sub.2 O.sub.3 and metal Fe. In order to greatly increase the recording density, a strong demand has recently arisen for new, fine magnetic powders. Extensive studies have been made for a new magnetic recording medium using a fine hexagonal barium ferrite powder, and a fine hexagonal barium ferrite powder was found to be suitable for high-density recording. The reason why hexagonal barium ferrite powder is used is that this ferrite powder consists of platelet-like particles, and axis of easy magnetization is perpendicular to the platelet planes of the ferrite particles, so that perpendicular orientation can be easily achieved by a magnetic or mechanical orientation process. The fine hexagonal ferrite magnetic powder is mixed with a binder resin and solvent, and then the mixture is coated on a surface of a nonmagnetic tape. The coated layer is placed in a magnetic field so that the surface of the layer is perpendicular to the direction of the magnetic field, thereby readily aligning the axis of easy magnetization of each fine magnetic particle with the direction of the magnetic field. Thereafter, the layer is dried to obtain a recording medium suitable for perpendicular magnetic recording.
Magnetic recording media have been recently used in a variety of applications. It is accordingly very important to improve reliability such as weather resistance and durability. Particularly, in the case of home video recorders, a relative speed between a reproducing head and a recording medium is about 7 m/sec. Under these conditions, high wear resistance is required for magnetic recording media. Such wear resistance must be obtained in temperatures varying from -50.degree. C. to 80.degree. C. with humidity ranging from 0% to 100%, and in seaboard and city atmospheres.
Demand for improved wear resistance in magnetic recording media has increased year by year. Conventional barium ferrite magnetic recording media do not always satisfy such a demand.
Since barium ferrite contains barium as its basic element, barium ions which are not caught within barium ferrite particle crystals are possibly present in a barium ferrite fine powder. With a magnetic recording medium using fine barium ferrite particles wherein barium ions are present in an amount exceeding a predetermined amount, a cross-linking hardening of the medium layer is degraded. When this magnetic medium is placed for a long period of time in an atmosphere having a high temperature and a high humidity, barium salt is precipitated on the surface of the medium, thereby degrading recording/reproducing characteristics.